1. Field of the Invention
The present invention relates to an aspherical microstructure, such as an aspherical microlens array that is usable in fields of optoelectronics and the like, a mold or a master of a mold (in the specification the term “mold” is chiefly used in a broad sense including both a mold and a master of a mold) for forming an aspherical microstructure, a method for fabricating the aspherical microstructure, and so forth. In the specification, the term “aspherical” is used in a broad sense and includes “acylindrical”.
2. Description of the Related Background Art
A microlens array typically has a structure of arrayed minute lenses each having a diameter from about 2 to 3 microns to about 200 or 300 microns and an approximately-semispherical profile. The microlens array is usable in a variety of applications, such as liquid-crystal display devices, light receivers and inter-fiber connections in optical communication systems.
Meanwhile, earnest developments have been made with respect to surface emitting lasers and the like that can be readily arranged in an array form at narrow pitches between the devices. Accordingly, there exists a significant need for microlens arrays with narrow lens intervals and a large numerical aperture (NA).
Likewise, light receiving devices, such as a charge coupled device (CCD), have been reduced in size as semiconductor processing techniques develop and advance. Therefore, also in this field, the need for microlens arrays with narrow lens intervals and a large NA is increasing. In the field of such a microlens, a desirable structure is a microlens with a large light-condensing efficiency which can highly efficiently utilize light incident on its lens surface.
Further, similar desires exist in the prospective fields of optical information processing, such as optical parallel processing-operations and optical interconnections.
Furthermore, display devices of active or self-radiating types, such as electroluminescent (EL) panels, have been enthusiastically studied and developed. Highly-defined and highly-luminous displays have been proposed. In such a display, there is a heightened desire for microlens arrays with improved luminescence and visibility, a large area, a small lens size, and a large NA, and which can be produced at a relatively low cost.
In general, the spherical aberration of a lens is likely to increase as its NA is enlarged while its spherical profile is maintained. Therefore, the surface of the lens needs to be aspherical such that compensation for the spherical aberration can occur. Similarly, in the case of a microlens, the profile of the microlens should be aspherical to improve its light-condensing efficiency. The need to improve the light-condensing efficiency is caused by the downsizing of optical devices.
In such a situation, the need for methods for fabricating aspherical microlens arrays and aspherical microlens arrays produced by these methods is increasing.
A conventional method for fabricating aspherical microlens arrays has been proposed. In a mask layer formed on a flat surface, openings are formed at positions of individual lenses and their surrounding positions. Through those openings, parts of the flat surface are chemically etched to form a mother substrate. Then, a mold for forming an aspherical microlens array is fabricated by using the mother substrate (see Japanese Patent Application Laid-Open No. 5(1993)-150103). However, when an isotropic etching using a chemical reaction is employed, etching of the substrate into a desired profile cannot be achieved if the composition and the crystalline structure of the substrate vary even slightly. In addition, etching will continue unless the substrate is washed immediately after the desired shape is obtained. When an aspherical microlens array must be precisely formed, deviation from the desired shape caused by excessive or over etching should be considered.
Further, another conventional method for fabricating aspherical microlenses has been proposed. Lens mold elements are tentatively formed in a lattice array by a punch having the desired radius of curvature for the lens mold element. The desired radius of curvature is obtained beforehand. The surface height error of a lens mold element surrounded by other lens mold elements is measured radially from the lens mold element, and the error is substituted by a correction amount. The radius of curvature of the punch is corrected based on that correction amount by a numerical control (NC) abrasion machine. Then, a microlens mold with a desired aspherical surface is accurately formed using the corrected punch (see Japanese Patent Application Laid-Open No. 6(1994)-154934).
In this method, however, much time is required to form a large number of concave molds over a large area. Further, it is difficult to fabricate concave molds with a uniform profile on the surface of the substrate when the punch's durability is insufficient.
Furthermore, still another conventional method for fabricating aspherical microlens arrays has been proposed. First, a photosensitive resin is deposited on a substrate. Next, an array of cylindrical resin layers are formed, in accordance with the required lens number and the intervals of a lens array, by using photolithography. Then, the cylindrical resin layers are thermally processed to deform them into spherical resin layers. The substrate is dry-etched using the spherical resin layers as a mask, and a profile similar to the profile of the mask is transferred onto the substrate. Then, the thus-fabricated mother substrate is used as a mold (see Japanese Patent Application Laid-Open No. 7(1995)-104106). In this method, an aspherical microlens array can be provided when the dry etching is performed such that the etching rate of the substrate is greater than the etching rate of the mask.
In this method, however, it is difficult to fabricate an aspherical microlens array with a large fill-factor since the dry etching is conducted using disk-shaped resin masks and a flat portion remains on the etched mother substrate.